RESUMO
As global temperature shifts due to anthropogenic impacts, seasonal temperatures in shallow aquatic ecosystems are expected to increase. Previous studies on freshwater fishes that experience significant temperature changes during the annual seasons found pronounced physiological restructuring not observed in animals inhabiting more thermally stable environments. Studies evaluating mitochondrial bioenergetics in fish are often performed on animals acclimated to constant temperatures in the laboratory. However, natural habitats are much more complex. Fishes may experience substantial daily and seasonal variation in temperature, energy requirements and resource availability, which are impossible to emulate on acclimation studies. Our study explores the effects of these more complex natural environments on whole-organism thermal tolerance and mitochondrial bioenergetics in bluegill sunfish (Lepomis macrochirus), a native fish to the temperate zone of North America. Compensatory mechanisms and variations in physiological thresholds were observed in specimens acclimatized to the fall season compared to specimens acclimatized to spring and summer seasons. Somatic indices, such as relative weights and hepatosomatic indices, showed significant differences across seasons and critical thermal maxima significantly decreased in the cold acclimatized specimens. Liver mitochondria from L. macrochirus also showed significantly higher uncoupled proton conductance, cytochrome c oxidase (COX) activity, and reduced respiratory control ratios in individuals sampled in the colder season. These findings suggest that mechanisms regulating proton conductance and COX activity modulate mitochondrial function across seasons to sustain physiological fitness in ectotherms inhabiting shallow, inland aquatic habitats.
